1. Field of the Invention.
This invention relates to aircraft interior modules. More particularly, it refers to an aircraft interior made of molded honeycomb panels with a layer of damping material bonded within the panels and the panel mounted so as to avoid direct contact with the interior of the aircraft skin.
2. Description of the Prior Art.
Interior insulation systems for airplanes have been well known for a long period of time. Improvements on these systems such as shown in U.S. Pat. No. 3,740,905 have not emphasized sound absorbing properties. However, sound absorption is becoming more and more important because of advanced power systems for new aircraft.
The level of low frequency sound impinging on the fuselage of future propfan powered airplanes is predicted to be higher than the level of low frequency jet exhaust noise that impinged on the aft fuselage of turbojet powered 707 and DC8 airplanes. The level of structure borne noise from propfan power propagating as vibration through the structure of a fuselage may well exceed that experienced on existing turbofan powered airplanes such as 727's and DC9's with aft mounted engines.
In 1986, passengers in newly delivered medium to long range transport airplanes (and the airlines that operate those airplanes) were accustomed to the level of acoustical comfort typified by the noise levels in 737, 300 and MD 80 airplanes where the levels of engine noise and boundary layer noise were well below the level of noise from advanced propfan propulsion systems.
A major task of the manufacturers is to find a means to introduce the fuel efficiency of advanced propfan engines without degrading the level of acoustical comfort provided for the passengers and crew by current technology transports.
Many business jet airplanes have special requirements for low interior noise levels during cruise. The interiors of most business jet airplanes are not installed by the airplane manufacturer because of the great variety of custom designs which are offered for individual operators. Airplanes are delivered in a "green" condition to "completion centers" for installation of the interior. To achieve the desired low interior noise levels, the installer of the interior utilizes special techniques and various combinations of damping material, vibration absorbers, vibration isolators, massive septa, and sound absorbing material.
A conventional approach to achieve low cabin noise levels has been to install fiberglass blankets between the fuselage frames for high frequency noise reduction. Noise reduction at low frequencies is provided by a massive septum made from a sheet of loaded vinyl (lead vinyl) outboard of the interior trim panel.
Double wall concepts have been evaluated theoretically and experimentally for transport category airplanes and versions of the concept are incorporated in all passenger carrying aircraft. The skin of the airplane forms one of the two walls; the interior sidewall trim panels and the ceiling panels form the other wall. A unified approach to a double wall has not been developed for transport category airplanes.